The present invention relates to packaging film and, more particularly, to an oriented, multilayer film with improved overlap heat sealability.
Polyvinyl chloride (PVC) has long been used by retail food suppliers as an overwrap material to package food products such as tray-mounted cuts of meat. Such products are typically packaged by placing the food product in a foam tray, positioning a piece of PVC overwrap film over and around the tray-mounted product, gathering and overlapping the ends of the film at the underside of the tray, and then placing the underside of the tray on a hot plate until the overlapped ends of the overwrap material become tacky enough to adhere to one another, thereby forming an overlap heat seal.
One disadvantage of PVC is that hydrogen chloride gas is often produced during the heat sealing process, thereby causing corrosion to occur in the packaging room. As a result, alternatives to PVC overwrap material have been sought.
One such alternative is described in U.S. Pat. Nos. 5,158,836 and 5,219,666. Those patents disclose an oriented, multilayer film having a very low density polyethylene (VLDPE) core layer with outer layers of a styrene butadiene copolymer (SBC). Such a film has excellent optical properties (i.e., clarity and gloss) and elasticity. Elasticity is desirable because it allows the film to return to its original shape after being handled, e.g., "poked," by customers seeking to inspect the packaged product, and results from the fact that the film is stretch-oriented during production. As is conventionally understood, the phrase "stretch-oriented" as used herein refers to a polymer-containing material which has been stretched at an elevated temperature (the orientation temperature), followed by being "set" in the stretched configuration by cooling the material while substantially retaining the stretched dimensions. Upon subsequently heating a stretch-oriented, polymer-containing material, such material will generally shrink in size (assuming it is unrestrained and unannealed). When heated to its orientation temperature, the material will shrink almost to its original (pre-oriented) dimensions.
As a result of being stretch-oriented, the foregoing multilayer film had a tendency to distort in the area of the overlap heat seal because of heat shrinkage. Such distortion is sometimes referred to as "shrinkback." Distorted heat seals are unsightly and prevent the packaged product from lying flat without wobbling, both of which are undesirable characteristics of a packaged product which is displayed for customer purchase.
Accordingly, it would be desirable to provide an oriented, multilayer film which is a suitable replacement for PVC as an overwrap material and which has improved overlap heat seal properties.